The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus capable of performing one-sided copying or two-sided copying on a transfer sheet.
There has so far been available an image forming apparatus wherein a toner image formed on an image forming object is transferred onto the obverse side of a transfer sheet, and an image formed on the image forming object is temporarily transferred onto a toner image receptor and then is transferred onto the reverse side of the transfer sheet, and the toner images thus transferred onto the transfer sheet are collectively fixed by a first fixing means and a second fixing means, so as to perform two-sided copying. There has been disclosed a technology to heat by independently setting temperatures respectively for the first fixing means and the second fixing means, for example, in the above-mentioned apparatus (Japanese Patent Publication Open to Public Inspection No. 171390/1991 (hereinafter referred to as Japanese Patent O.P.I. Publication)).
The invention further relates, in particular, to an image forming apparatus wherein toner images are formed on both sides of a transfer material, and these images are collectively heated and fixed for two-sided image formation.
In two-sided copying, the conventional technique has been a method wherein an image on one side of an original formed on an image forming object is transferred and fixed on a transfer material which is then stored temporarily in an intermediate tray, and the transfer material is fed out of the intermediate tray in synchronization with an image on the other side of the original formed subsequently on the image forming object, so that the image on the other side of the original may be transferred and fixed on the other side of the transfer material.
In the two-sided copying apparatus mentioned above, a transfer material is fed to the intermediate tray and is caused to pass through a fixing unit twice, as stated above. Therefore, a conveyance distance for a transfer material is long, which causes a long processing time for copying, and reliability for conveyance of a transfer material is low because a transfer material which has passed through the fixing unit once to be ready for curling is conveyed again to pass through the fixing unit, which has been a cause for jamming. To overcome the problems mentioned above, there have been proposed technologies to form toner images on both sides of a transfer material and to fix them at the same time in Japanese Patent Examined Publication Nos. 37538/1974 and 28740/1979, and Japanese Patent O.P.I. Publication Nos. 44457/1989 and 214576/1992.
In ordinary image formation, however, frequency of image forming on one side of a transfer material (one-sided copy mode) is high, and when copies are made continuously, a temperature of a fixing roller is gradually lowered. In this case, a temperature of a fixing roller facing the opposite side (rear side) of the surface on which toner images are formed is also lowered because a transfer material passing through the fixing unit takes heat away. Therefore, when image formation on both sides of a transfer material (two-sided copy mode) is designated, and two-sided copying is executed without taking any action, insufficient fixing for images on the reverse side is unavoidable. Due to that, toner images fixed imperfectly are ejected out, soiling the hands of a user and soiling other copies with toner images moved to other transfer materials, which is a problem.
A first object of the invention is to solve the problem mentioned above.
Since the fixing roller of the first fixing means and that of the second fixing means in the prior art are independently subjected to temperature setting and are independently heated, when both fixing rollers are used in two-sided copying, after heating the fixing roller on the side facing visible images on a one-sided copy in one-sided copying, for example, heating efficiency is poor and heating time is long because the fixing roller is heated independently.
The invention has been achieved in view of the aforesaid problem, and its second object is to provide an image forming apparatus wherein the heating time for a fixing roller is short, heating efficiency is excellent, and idling time for fixing is made to be shortest in each of one-sided copy mode and two-sided copy mode, or idling time for fixing in two-sided copy mode and that in one-sided copy mode are standardized.
However, it is not easy to collectively fix a transfer material having toner images on its both sides. For the purpose of fixing collectively the transfer material having toner images on its both sides, it is necessary to arrange heating means over both sides of the transfer material, and power consumption at the fixing means is inevitably high because power needs to be supplied to both heating means. However, a reduction of power consumption has not heretobefore been proposed. Further, an increase in power consumption causes a rise in temperature of an apparatus, resulting in occurrence of a change in characteristics of a photoreceptor and other troubles, thus, excellent images can not be formed and an operation expense for the apparatus is increased, which has been a problem.
A third object of the invention is to provide an image forming apparatus wherein the problem mentioned above has been solved, and both a transfer material having toner images on its both sides and a transfer material having a toner image on its one side can be fixed excellently with less power consumption.